Radio propagation within a cell must meet two essential requirements, namely transmission at less than excessive power by the base station and reception of sufficiently powerful signals at the terminals.
In the first place the range of each base station must be adequate to extend into an adjacent cell in order to preclude any danger associated with a loss of contact that occurs when a mobile terminal moves into another cell. Accordingly the power at the transmitter must exceed a rigorous minimum.
In the second place, because the radio links propagate substantially straight at ground level, the station's shadow zones caused by the local topography or by buildings must also be covered. A shadow zone is a zone wherein the attenuation of radio propagation between a mobile terminal therein and a station falls below a specified sensitivity for radio circuits, whereby a received level is inadequate to properly detect transmitted bit packets representing voice or data to be exchanged. On the other hand, the levels of transmitted power cannot be increased.
The reason is that, at the base station, any increase in power also would increase cell size, entailing undue interference between adjacent cells. Maximum power at the mobile terminals is limited on safety grounds and the operating time of charged batteries at the mobile terminals.
Furthermore the number of base stations or repeaters must not be needlessly multiplied to take care of the microcells—the shaded areas—so costs and interferences can be managed.
In the prior art, the attenuations are ascertained at a plurality of sites in a cell by using a vector database from the French National Geographic Institute (IGN) for instance, which represents a map of the geographic zone under consideration. The map contains buildings and other above-ground structures. Different code words define the kind of above-ground structures, for instance woods, lodgements, water, which are specified in Lambert coordinates and height above the local ground altitude relative to the sea.
The transmission of radio signals is simulated to provide an anticipatory estimate of the attenuation at any point within the cell. Propagation is modeled by computing a set of radio coverages in a computer as an electromagnetic beam centered on the base station. The propagation extends in a given direction, and the cell propagation conditions are then calculated within the element of a solid angle subtended by the beam. Except for the propagation in free space when the station is in direct line of view with a simulated radio terminal—from which follows a known linear propagation attenuation—the beam path strikes obstacles which attenuate it or additionally deflect it, in particular within micro-cells where the stations are situated at an altitude lower than the buildings' roofs.
Illustratively, in a street, the beam can deviate by reflection or refraction. As a result the aperture of its solid angle can increase.
Such calculations are repeated for a plurality of elementary solid angles distributed within a global solid angle of view of the entire cell, for instance a substantially horizontal annulus, in order to sample the various propagation conditions of the cell's space.
At each point of each beam's path, an operator consults the vector database to determine whether an obstacle is present. The related vectorial calculations require considerable computing power and might entail a day's work, and in practice the calculations must be initiated during the evening when conventional computers are used.
The objective of the present invention is to reduce the computing power required to calculate the radio coverage of such cells, whether the cells be large, i.e macro-cells, or microcells.